1. Field of the Invention
The present invention relates to the field of microcomponents and microsystems, and more specifically to methods of deep anisotropic etching of a silicon wafer.
2. Discussion of the Related Art
Two main methods for anisotropically etching wells, trenches or ribs (mesas) in silicon wafers are industrially used.
A first so-called alternated method is for example described in U.S. Pat. No. 5,501,893 of Bosch Company.
As illustrated in FIGS. 1A to 1D, the alternated method comprises a repeated succession of etch and polymerization steps which are implemented at a temperature close to the ambient temperature.
FIG. 1A is a partial view of a silicon wafer 1 coated with a protection layer 2, for example, a silicon oxide layer in which an opening 3 has been formed, for example, a circular hole or a strip of small width. The silicon wafer is placed in a plasma reactor (not shown) and a first recess 3 is etched by plasma in the trench. For this etching, a fluorinated compound, for example, sulfur hexafluoride, SF6, is injected into the plasma reactor.
At the next step, illustrated in FIG. 1B, a gas such that the resulting active components in the reactor are likely to form a polymer 4 on the silicon walls of recess 3 is introduced into the plasma reactor. This polymer forming gas is for example trifluoromethane, CHF3 or C4F8. The formed polymer is a film of a material that can be assimilated to Teflon (CF2)n.
In a next etch step illustrated in FIG. 1C, the fluorinated plasma first etches by ion bombarding the polymer layer at the bottom of recess 3, then forms a complementary isotropic etch 5.
By repeating the above steps, an etch 7 is obtained in silicon wafer 1, as illustrated in FIG. 1D.
The second so-called cryogenic method for anisotropically etching a silicon wafer is illustrated in FIG. 2 which shows a silicon wafer 1 coated with a hard mask 2, for example, made of oxide. The wafer is placed in a plasma reactor on a susceptor cooled down to a very low temperature, for example, around −100° C., and a plasma etch in the presence of sulfur and oxygen hexafluoride is then performed. The method is continuous, that is, there are no alternated steps of etching and deposition of a protection layer on the walls of the recess being formed. With this method, a passivation layer 9 of SiOxFy type, that is, a saturated material, not likely to form a polymer, forms on the recess walls.
Each of these two conventional methods has advantages and disadvantages.
A disadvantage of the structure obtained by the alternated method is that the walls of the obtained well, trench or rib are grooved while they are smooth with the cryogenic method.
Another disadvantage of the alternated method with respect to the cryogenic method lies in the forming of a polymer on the recess walls. This polymer is difficult to remove, which makes the method poorly adapted to certain applications in electronics where the quality of the contact with the trench walls is particularly important. This polymer not only deposits in the formed recess but also on the reactor walls, which obliges to frequently cleaning this reactor and causes an efficiency decrease and method drifts. However, with the second method, deposited material 9 becomes gaseous when the wafer is brought back from the deposition temperature (approximately −100° C.) to the ambient temperature and eliminates by itself.
Another disadvantage of the alternated method lies in the fact that, in given plasma conditions, the etchings are substantially twice as slow, i.e. half as fast as with the cryogenic method.
The cryogenic method is better adapted to the forming of patterns of small extent with respect to the size of the wafer to be processed (opening rate smaller than 20%) and at short etch times (shorter than one hour), that is, in cases where a small amount of matter is to be removed. Conversely, the alternated method is better adapted to micromechanical applications, of MEMS type, for which there often is a large amount of matter to be etched.
The alternated method has the advantage of a reliable operation, that is, once the plasma conditions have been set, the etch rate and the shape of the obtained opening are well reproducible even if the operating parameters vary a little. Conversely, the cryogenic method has the disadvantage of being very sensitive to the operating parameters and especially to temperature. When the temperature varies slightly, for example, by approximately 1° C., either from one area of a wafer being processed to another, or for two successively-processed wafers, there result variations in the etch rate and in the shape of the etched recess. Especially, if the conditions are poorly set, one tends to have an opening which, instead of having properly vertical walls, exhibits a conical shape, the conicity being directed downwards or upwards according to whether the temperature is too high or too low.
Another advantage of the alternated method is that its implementation causes a relatively light underetch under mask 2, for example, on the order of 0.2 μm. However, by this prior cryogenic method, a relatively significant underetch, on the order of one μm, is obtained.
Another advantage of the alternated method is its simplicity of implementation, since it is not necessary to provide means for cooling the processed wafer.
A third method is described in U.S. Pat. No. 6,303,512 of Bosch Company. However, unlike the two previously-described methods, this third method has, to the present applicants' knowledge, had no industrial application. The third method provides, in one of its many alternative embodiments, a plasma etch at ordinary or slightly lower temperature (the given example is +10° C.) by sulfur and oxygen hexafluoride, and in the simultaneous or alternated presence of silicon tetrafluoride, SiF4. The oxygen and silicon tetrafluoride flow rates in the plasma enclosure are of the same order of magnitude as the SF6 flow rate, or even greater (see claims 3, 17, and 18). The etch and passivation phases may be alternated.
The present applicants have carried out tests by attempting to reproduce the conditions described in U.S. Pat. No. 6,303,512, without injecting any C4F8. The present applicants have found, in particular, that the application of the method of this patent would result in a blocking of the silicon etch, and in all cases to a very slow etching with respect to the etch rates of the two previously-described methods. Further, as indicated by the patent, there forms on the opening walls silicon nitride or oxide, which is difficult to eliminate.
Further, the present applicants have tried the method of U.S. Pat. No. 6,303,512, at cryogenic temperatures, lower than −40° C. (not suggested in the patent) and have noted, in this case as well, a complete blocking of the etch process.